This invention relates to the field of public health, and particularly to a new clinical thermometer arrangement in which the necessity to "shake down" the mercury after a reading has been taken is obviated.
As is well known, a clinical thermometer is a device designed for taking a patient's temperature by being inserted into a body cavity and allowed to come to a condition of thermal equilibrium therewith. The instrument comprises a closed transparent tube, usually of glass, having external graduations. The bore of the tube is connected at one end with a reservoir of thermometric fluid, that is, fluid having a much greater coefficient of thermal expansion than that of the glass. The quantity of liquid is such that when the temperature of the reservoir or bulb varies within a predetermined range a meniscus of the liquid, usually mercury, moves within the tube and contiguous to the graduations. The bore of the tube is made of small size, compared to the reservoir, so that a small temperature responsive volume change in the mercury may cause a very perceptible movement of the meniscus in the tube. Herein this bore dimension will be referred to as capillary.
When the thermometer is removed from the body cavity into the lower ambient temperature, for reading, the mercury will immediately begin to move down with respect to the scale, so that the reading observed might be perceptibly lower than the maximum actually present. To prevent this, it is known to provide a restriction in the bore of the thermometer, reducing its size very considerably.
It will be appreciated that movement of the mercury out of the reservoir as the temperature rises is due to expansion of the mercury and is a very significant force: as a matter of fact thermometers are often provided with a second bulb at their other ends to receive mercury and prevent breakage of the thermometer by expanding mercury if inadvertenty exposed to high temperatures. On the other hand, when the temperature falls and the mass of mercury contracts, that portion of the mercury in the bore is impelled back into the reservoir principally by the cohesiveness of the mercury itself. The end of the tube remote from the reservoir is either evacuated or charged with an inert gas at low sub atmospheric pressure: in either case the force exerted on the mercury meniscus is not great, and the same is true as regards the surface tension forces at the meniscus. The restriction offers such resistance to the passage of mercury that the cohesiveness of the mercury is overcome and the column of mercury separates at the restriction, so that the maximum reading of the meniscus is retained. This leaves however, the problem of resetting the thermometer for reuse after its reading has been recorded or observed.
The traditional way of resetting a clinical thermometer is known as "shaking down" the thermometer, and is an acquired skillful motion of the arm and wrist which overcomes by centrifugal force the resistance of the restriction, and drives the mercury from the bore to the reservoir. Even among professional health workers this is an annoying necessity and occasionaly a difficult one: for private individuals where a home thermometer is only seldom used it becomes a major disadvantage of and deterent to free use of the instrument.